1. Field of the Invention (Technical Field)
The present invention relates to short circuit protection mechanisms and more particularly short circuit protection in general high and low side voltage driver circuits implemented in an integrated circuit""s output driver.
2. Background Art
There has been a need for a minimal impact load short circuit protection apparatus for a high and low side driver that does not affect or degrade the driver output performance. The present state of the art calls for expensive solutions, and due to increased number of components, the solutions take up an inordinate amount of circuit area.
Most circuits use a series current sense resistor, and disable the driver if the voltage developed across the resistor exceeds a predetermined value. This is sometimes done in an analog fashion such as with a series emitter resistance that cuts back the current or done digitally using a comparator. The current sense resistor can be costly, depending on performance required, and generally penalizes the design by increasing power dissipation, saturation voltage, output resistance and decreasing in-rush current capability, rise/fall times, and reliability.
There are several prior art drivers that are able to withstand short circuit conditions and include this expensive solution. The device disclosed in U.S. Pat. No. 4,698,582 eliminates the need for a current sense resistor, but measures the output transistor""s base-emitter voltage (instead of collector-emitter voltage) and uses costly comparator, reference, and latching circuitry. The following patents also disclose using a series resistor: U.S. Pat. Nos. 6,008,972, 5,757,601, 5,696,658, 5,457,364, 4,771,357 and U.S. Pat. No. RE33,941. U.S. Pat. No. 5,432,665 teaches using multiple pass transistors with different amounts of series resistance. A low resistance transistor is used for initial switching, and the high resistance transistor is used if the output state does not change (i.e. short circuit). U.S. Pat. No. 4,945,358 uses a parallel resistor and inductor, impacting the driver""s performance, including cost as well as leakage current in this case. The device disclosed in U.S. Pat. No. 4,394,148 does not turn off during a short circuit. The pass transistor is sized to handle short circuit conditions.
State of the art approaches are more costly and/or degrade driver performance. However, none of the prior art devices protect electronic voltage driver circuits from output loads shorting while using only a small amount of circuit area. The prior art devices also significantly degrade driver output performance, which include output resistance, saturation voltage, in-rush current capability, output rise and fall times, output voltage transient immunity, power dissipation, output leakage current, and reliability.
In accordance with the present invention, disclosed is method and apparatus for providing short circuit protection for a low side or high side driver without substantially impacting the driver""s performance. This circuit provides short circuit protection for short circuits to ground or to VCC.
The preferred short circuit protection apparatus for an output of a high or low side driver comprises a means for constantly monitoring an output voltage, a means for comparing the monitored output voltage to a known reference voltage, a switching means for switching the output to a predetermined voltage after an actuation delay if the constantly monitored output voltage equals or exceeds the known reference voltage after said actuation delay and a delay circuit for providing said actuation delay for a period longer than a capacitive output fall time. The preferred switching means comprises a pass transistor. The predetermined voltage can be ground or VCC. The preferred means for comparing comprises a high input voltage comparator. The preferred delay circuit comprises a current source delay circuit. The preferred means for constantly monitoring and comparing the output voltage comprises a diode. The diode preferably comprises an end connected to an output.
The preferred short circuit protection apparatus for an output of a high or low side driver comprises a diode for constantly monitoring and comparing an output voltage, a comparator comprising the diode for comparing the monitored output voltage to a known reference voltage, a switch driven by the comparator for switching the output to a predetermined voltage after an actuation delay if the constantly monitored output voltage equals or exceeds the known reference voltage after the actuation delay, and a delay circuit for providing the actuation delay for a period longer than a capacitive output fall time.
The preferred method of protecting a high or low side driver from output short circuits comprises the steps of monitoring an output voltage, comparing the monitored output voltage with a known reference voltage, activating a switch to pull the output to a predetermined voltage after an actuation delay if the monitored output voltage equals or exceeds the known reference voltage and providing the actuation delay with a source delay circuit for a period longer than a capacitive output fall time. The preferred step of activating a switch comprises activating a pass transistor. The step of activating a switch to pull the output to a predetermined voltage comprises pulling the output to ground or to VCC. The preferred step of comparing comprises comparing the monitored output voltage with a known reference voltage using a high input voltage comparator. The preferred step of providing an actuation delay comprises using a current source delay circuit. The preferred step of monitoring and comparing the output voltage comprises monitoring and comparing the output voltage with a diode. The preferred step of monitoring an output voltage comprises constantly monitoring the output voltage.
A primary object of the present invention is to provide inexpensive short circuit protection to a high or low side driver without affecting driver output performance.
Another object of the present invention is to provide short circuit protection to a high or low side driver with as few components as necessary.
A primary advantage of the present invention is that it is inexpensive compared to other present systems.
Another advantage of the present invention is that it uses only a small amount of circuit area due to the elimination of components.
Yet another advantage of the present invention is the reduction of total circuit power dissipation due to the reduced number of components.
Other objects, advantages and novel features, and further scope of applicability of the present invention will be set forth in part in the detailed description to follow, taken in conjunction with the accompanying drawings, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.